Dichotomous branching: the plant form and integrity upon the apical meristem bifurcation

Abstract

The division of the apical meristem into two independently functioning axes is defined as dichotomous branching. This type of branching typically occurs in non-vascular and non-seed vascular plants, whereas in seed plants it presents a primary growth form only in several taxa. Dichotomy is a complex process, which requires a re-organization of the meristem structure and causes changes in the apex geometry and activity. However, the mechanisms governing the repetitive apex divisions are hardly known. Here, an overview of dichotomous branching is presented, occurring in structurally different apices of phylogenetically distant plants, and in various organs (e.g., shoots, roots, rhizophores). Additionally, morphogenetic effects of dichotomy are reviewed, including its impact on organogenesis and mechanical constraints. At the end, the hormonal and genetic regulation of the dichotomous branching is discussed.

Keywords: apical cell; apical meristems; dichotomy; meristem bifurcation; terminal branching.

FIGURE 1

Mechanisms of dichotomous branching. (A) Dictyota dichotoma type: an equal longitudinal division (marked with a dashed line) of the single apical cell. (B) Inactivation of the original apical cell (crossed triangle) followed by the simultaneous initiation of the branch initials (two triangles next to the original apical cell). (C–E) Dichotomy in the meristems with one (C) or more (D,E) groups of initial cells. The meristem zonation is maintained during dichotomy, but the number of cells increases due to intensive cell proliferation. Centrally located cells of the original meristem start to differentiate (crossed groups of cells), separating the dichotomous apices at the flanks of the original meristem. C,D – shoot apices, E – roots.